One apparatus for measuring the frequency distribution of an input signal is a spectrum analyzer. The configuration of a circuit that can be used as the detector of a spectrum analyzer is shown in Patent Document 1: it detects the amplitude and phase of the input signal by using a complex sine wave generated by a local oscillator to down-convert the band-limited input signal and removes unwanted high frequency components with a low-pass filter. A similar configuration is seen in Patent Document 2, where it is used as part of a demodulator that detects the amplitude of a modulating signal from a high-frequency signal generated by amplitude modulation. Amplitude modulation uses a carrier signal to up-shift the unaltered frequency distribution of the modulating signal, so the demodulator can be regarded as a circuit that detects the input signal by down-shifting its frequency distribution by an amount equal to the carrier frequency.
There are cases in which what is wanted is not to detect the entire frequency distribution of the input signal, but to determine automatically whether the input signal includes a particular frequency component. For example, it may be necessary to determine what color television system is being used in an analog broadcast. The color subcarrier frequency in the NTSC system used in Japan and the United States of America is approximately 3.58 MHz (the precise value is 3.579545 MHz); the PAL system used in Europe has a different color subcarrier frequency of approximately 4.43 MHz (the precise value is 4.43361875 MHz). In the SECAM system the color subcarrier frequency switches between 4.25 MHz and 4.40625 MHz once per horizontal interval. A multi-standard color television receiver therefore requires a circuit that discriminates the color subcarrier frequency of the input video signal.
In color television transmission, depending on the channel characteristics, the amplitude of the color subcarrier may be greatly attenuated, or much noise may be present. Just by measuring the signal amplitude or signal power at each frequency with the same configuration as a spectrum analyzer and adding a circuit to evaluate the magnitude of the measured value of a particular frequency component, accordingly, it would not necessarily be possible to identify the color subcarrier frequency of the input video signal correctly.
In Patent Document 3 there is an exemplary circuit that determines whether the color subcarrier frequency is 3.58 MHz or 4.43 MHz, using a trap filter with a stop-band around 4.43 MHz. It extracts the color burst signal that forms the color subcarrier frequency reference from the input video signal, compares the color burst signal with a signal obtained by passing the color burst signal through the trap filter, and determines the color subcarrier frequency of the input video signal to be 4.43 MHz if it is attenuated by the trap filter, or 3.58 MHz if it is not attenuated. A feature of this method is that the frequency discrimination result does not depend on the amplitude of the color burst signal, but when the noise amplitude is large, there is the possibility of incorrect frequency discrimination, depending on whether or not the noise component is attenuated by the trap filter.
In Patent Document 4 there is an exemplary circuit that determines whether the color subcarrier frequency changes at each horizontal interval, and uses the result of this determination to recognize whether or not the input video signal is a SECAM signal. This circuit makes use of the property that the phase delay of an RLC resonant circuit is approximately +90 degrees on the high-frequency side and approximately −90 degrees on the low-frequency side. Since discrimination is not affected even if the resonant frequency of the resonant circuit is slightly displaced, the 4.40625-MHz (or 4.25-MHz) color subcarrier frequency is down-converted to 654 kHz (or 810 kHz) by use of a 5.06-MHz complex sine wave, and the resonant frequency of the resonant circuit is 732 kHz. If the real component of the down-converted signal is passed through the resonant circuit and multiplied by the imaginary component, whether the frequency of the input signal is higher or lower than a reference frequency (5.06 MHz−732 kHz=4.328 MHz) can be determined by detecting whether the product is positive or negative.
The features of this system are that the frequency discrimination result does not depend on the amplitude of the color subcarrier and is comparatively immune to noise, but it is not suited for applications that must discriminate large frequency differences. To maintain accurate frequency discrimination when the frequency difference is small in relation to the resonant frequency, it is necessary to use a resonant circuit with a high Q value. If a resonant circuit with a high Q value is used, however, then an input signal differing greatly from the resonant frequency will be unable to pass through the resonant circuit, and the product will be close to zero regardless of the frequency difference. Since the product value is close to zero even when the input signal frequency is close to the resonant frequency, it is difficult to discriminate an input signal with a frequency that actually differs greatly from the resonant frequency correctly. If used as is, this SECAM discrimination circuit cannot accurately determine whether the color subcarrier frequency is 3.58 MHz or 4.43 MHz.    Patent Document 1: U.S. Pat. No. 4,594,555 (pp. 21-22, FIG. 6)    Patent Document 2: U.S. Pat. No. 4,090,145 (pp. 10-11, FIG. 4)    Patent Document 3: Japanese Patent No. 3500883 (p. 4, FIG. 1)    Patent Document 4: Japanese Patent Application Publication No. H10-051802 (p. 3, FIG. 1)